Phase separation is when a combined liquid separates into two different liquids and may occur when autogas used for aviation fuel.
When alcohol present in autogas, it is subject to phase separation, when the fuel is cooled as a result of the aircraft’s climbing to higher altitude. Think of an oil and vinegar salad dressing; when the dressing is shaken, the oil and vinegar combine. As the salad dressing sits, the oil and vinegar separate. A similar situation exists when autogas with ethanol is combined with water. The presence of ethanol in autogas allows an amount of water to be absorbed,
rather than remaining separate.
There is a limit to how much water can be absorbed by autogas with ethanol, and the limit is dependent on the amount of ethanol in the autogas and the temperature of the fuel. If the water that has been absorbed by the autogas with ethanol separates, a layer of water and ethanol will form below the autogas. This water and ethanol blend is not the fuel an airplane engine likes to burn, so at least a partial, if not complete, power loss may occur. At 60°F, approximately 0.6 ounces of water can be absorbed by a one-gallon blend of 90 percent gasoline and 10 percent ethanol before the water will begin to phase separate. If the temperature cools to around 32°F, 20 percent of the total water present in the fuel will separate. On a 48-gallon fuel system, this means approximately six ounces of water could collect and go through the fuel system. The certification rules only require a sediment bowl (gascolator) capacity of one ounce for every 20 gallons. It doesn’t take a degree in advanced math to see that this situation could lead to a problem
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